1. Field of the Invention
This invention relates to the process of fabricating semiconductor chips. More specifically, the invention relates to a method and apparatus for registering a database image against a noisy scanned image, such as an image taken using a Scanning Electron Microscope.
2. Related Art
The relentless miniaturization of integrated circuits has been a key driving force behind recent advances in computer technology. Today, integrated circuits are being built at deep sub-micron (DSM) dimensions. At these dimensions, photomask accuracy is becoming increasingly important in the chip manufacturing process.
A photomask is typically a high-purity quartz or glass plate that contains deposits of chrome metal, which represents the features of an integrated circuit. The photomask is used as a “master” by chipmakers to optically transfer these features onto semiconductor wafers.
The mask-making process involves complex physical, transport, and chemical interactions. As a result, the actual photomask is different from the “perfect” photomask. If this difference is too large, it can render the photomask useless. Hence, it is important to measure the features of the actual photomask, so that one can ensure that the difference is within the error tolerance.
Before the features on the actual photomask can be measured, the photomask needs to be scanned using an imaging process to generate a scanned image. Furthermore, the scanned image must be accurately aligned with the perfect image so that the features on the scanned image can be accurately measured. This alignment process is called “photomask image registration”.
Typically, photomask image registration only involves determining the translational differentials (in 2D space) between the scanned image and the perfect image.
Unfortunately, the scanned image can contain noise, which can sometimes be excessive, e.g., the noise in a Scanning Electron Microscope (SEM) image. Furthermore, the scanned image and the perfect image can have different pixel depths. Moreover, the scanned image can contain a large number of pixels. For example, the scanned image can be a 512×512 grayscale image with an 8-bit pixel depth. For these reasons, it is very difficult to accurately and efficiently compute the translational differentials between the scanned image and the perfect image of a photomask, especially when the scanned image has low signal-to-noise ratio (as in SEM images).
Consequently, image registration for noisy images (e.g., SEM images) is typically performed manually. Unfortunately, this manual step slows down the chip fabrication process, which increases the cost and the time taken to manufacture chips.
Hence, what is needed is a method and apparatus for accurately and efficiently computing the translational differentials between the scanned image and the perfect image of a photomask.